Abstract
Abstract Measurements and mechanical models of heterogeneous bedload transport in rivers remain basic challenges for studies of landscape evolution and watershed management. A 700 m reach of the Trinity River (northern California, USA), a large gravel-bed river, was instrumented with an array of 76 seismographs during a dam-controlled flood and gravel augmentation to investigate the potential for out-of-stream monitoring. The temporal response to gravel augmentation during constant discharge provides strong evidence of seismic sensitivity to bedload transport and aids in identification of the seismic frequencies most sensitive to bedload in the study area. Following gravel augmentations, the seismic array reveals a period of enhanced transport that spans most or all of the reach for ∼7–10 h. Neither the duration nor the downstream extent of enhanced transport would have been constrained without the seismic array. Sensitivity to along-stream transport variations is further demonstrated by seismic amplitudes that decrease between the upper and lower halves of the reach consistent with decreased bedload flux constrained by time-lapse bathymetry. Insight into the magnitude of impact energy that reaches the bed is also gained from the seismic array. Observed peak seismic power is ∼1%–5% of that predicted by a model of saltation over exposed bedrock. Our results suggest that dissipation of impact energy due to cover effects needs to be considered to seismically constrain bedload transport rates, and that noninvasive constraints from seismology can be used to test and refine mechanical models of bedload transport.
Highlights
Mechanical work done at Earth’s surface by erosional processes can contribute to the signals recorded by ground velocity sensors in seismographs
Seismic detection of coarse sediment transport has been suggested based on hysteresis in observed seismic power versus water discharge, which likely represents variable bedload transport for a given discharge owing to temporally evolving bedload supply (Burtin et al, 2008; Hsu et al, 2011; Schmandt et al, 2013; Díaz et al, 2014; Roth et al, 2014; Chao et al, 2015)
The utility of detecting bedload by observing hysteresis is limited because all bedload transport events need not exhibit hysteresis and changes in bed morphology could contribute to hysteresis by changing turbulent flow (Gimbert et al, 2014)
Summary
Mechanical work done at Earth’s surface by erosional processes can contribute to the signals recorded by ground velocity sensors in seismographs. Physical sampling was conducted with a Toutle River 2 sampler using an average of 11 spaced placements across the transect to estimate the bedload transport rate and grain-size distribution 5–6 times each day (Fig. 1; see the Data Repository). The highest amplitudes were observed between the gravel injection site and ~250 m downstream, where migrating ~0.2–0.4-m-tall bedforms were detected by differential bathymetry (Fig. 3A) This amplitude pattern was similar for the first night of peak discharge and the three night mean. The lower bound on bedload flux from time-lapse bathymetry suggests a factor of ~5 downstream decrease in gravel transport from segment 1 to 2, consistent with bedload flux as a cause of alongstream seismic amplitude variations. Net accumulation of gravel is likely because the study reach slope of 0.0012 is lower than the average slope of 0.0023 between the dam and the study area
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
